The matching of sensor signals is needed in many applications where multiple versions of the same signal or signals are gathered. As a result of the natural variations within any device or system, the sensitivity of individual sensors differs each from the other and therefore the resulting electrical output signals may not be the same even though they have the identical input signal. Similarly, there are natural variations in the multiple signal handling electronics, like the sensor signal pre-conditioning circuits, that can add more differences to what should be identical signals. Multi-sensor or sensor array applications span the range from medical diagnostic imaging systems (ultrasound imagers, MRI scanners, PET scanners), to underwater sonar systems, to radar, to radio and cellular communications, to microphone systems for gunshot detection or voice pick up.
Multi-sensor sound pickup systems are becoming more common as the performance limitations of single microphone systems, especially in high noise situations, are rapidly being approached. Multi-microphone systems offer significantly improved performance capabilities, and therefore are to be preferred for use, particularly in mobile applications where the operating conditions can not be predicted. For this reason, multiple microphone pickup systems, and the associated multi-microphone signal conditioning processes, are now being used in numerous products such as Bluetooth® headsets, cellular handsets, car and truck cell phone audio interface kits, stage microphones, hearing aids and the like.
Numerous systems have been developed that depend upon microphone arrays for providing multiple spatially separate measurements of the same acoustic signals. For example, in addition to the well known beam forming methods, there are now generalized sidelobe cancellers (GSC), blind signal separation (BSS) systems, phase-based noise reduction methods, the Griffiths-Jim beamformer, and a host of other techniques all directed at improving the pick up of a desired signal and the reduction or removal of undesired signals.
However, along with the benefits of multiple microphone pickup systems come new challenges. One major challenge is that to achieve the performance potential of such systems requires that the sensors' signals be well-matched, a process often called “microphone matching.” This is because, depending upon the specifics of the system, magnitude mismatches, phase mismatches or both may severely degrade performance. Although the tolerance for microphone mismatch of each of these systems varies, most are quite sensitive to even small amounts of mismatch.
In many applications, even well-matched microphone elements will have significantly different response characteristics once mounted in microphone housings and placed or worn in the manner intended for the application. Even user-dependent variables can have substantially differing impact on the response characteristics of the individual microphones of a microphone array.
Another concern with multiple microphone systems is manufacturability. Pre-matched microphones are expensive and can change characteristics with time (aging), temperature, humidity and changes in the local acoustic environment. Thus, even when microphones are matched as they leave the factory, they can drift in use. If inexpensive microphones are to be used for cost containment, they typically have an off-the-shelf sensitivity tolerance of ±3 dB, which in a two-element array means that the pair of microphones can have as much as a ±6 dB difference in sensitivities—a span of 12 dB. Further, the mismatches will vary with frequency, so simple wide band gain adjustments are usually insufficient to correct the entire problem. This is especially critical with uni-directional pressure gradient microphones where frequency-dependent mismatches are the rule rather than the exception.
What is needed to make such systems perform at their highest level is an automatic, robust, accurate and rapid acting sensor sensitivity difference correction system, sometimes called a sensor matching system, capable of performing frequency dependent, real time matching of multiple sensor signals.